S = input()

alp = "abcdefghijklm"

n = 0
ans = None
for i in range(13):
    if S.count(alp[i]) == 0:
        n += 1
        ans = alp[i]
        
    if n > 1:
        break

        
if n == 0:
    for i in range(13):
        print(alp[i])
else:
    print(ans)